1. Field of the Invention
The invention relates to an injection system for an internal combustion engine, including a high-pressure fuel pump, a low-pressure pump for pumping fuel from a fuel tank to an intake side of the high-pressure fuel pump, and a relief device for lowering the pressure on a pressure side of the high-pressure fuel pump when the engine is switched off, or during the overrun fuel cutoff.
2. Description of the Prior Art
An injection system of the above type is known from German Patent Disclosure DE 195 39 883 A1. In this injection system, after the engine is turned off, a pressure equalization is established between the pressure side of the high-pressure fuel pump and the fuel tank, or the ambient pressure. This provision effectively prevents fuel from reaching the combustion chambers through the injection valves after the engine has been shut off. Because of this uncombusted fuel, there are increased emissions of uncombusted hydrocarbon compounds the next time the engine is started. Because of the incomplete pressure reduction from the pressure side of the high-pressure fuel pump and the tank to the ambient pressure, the pressure buildup adversely affects the starting performance of the engine.
The primary object of the invention is to improve the starting performance of this fuel injection system. According to the invention, in an injection system for an internal combustion engine having a high-pressure fuel pump, a low-pressure pump for pumping fuel from a fuel tank to an intake side of the high-pressure fuel pump, and a relief device for lowering the pressure on a pressure side of the high-pressure fuel pump when the engine is switched off, this object is attained in that the pressure on the intake side of the high-pressure fuel pump when the engine is off is greater than or equal to the delivery head of the low-pressure pump, and that the relief device, when the engine is switched off, establishes a pressure equalization between the pressure side and the intake side of the high-pressure fuel pump.
In the injection system of the invention, a partial pressure relief takes place on the compression side of the high-pressure fuel pump after the engine is shut off. However, the pressure is not reduced down to ambient pressure but instead, at most, to a pressure corresponding to the delivery head of the low-pressure pump. The delivery head of a low-pressure pump is typically between 3 and 6 bar, while on the pressure side of the high-pressure fuel pump, pressures of approximately 100 bar prevail during engine operation. Thus all the seals and sealing seats on the pressure side of the high-pressure fuel pump are relieved to such an extent by the partial relief of the invention that it is certain that no fuel can reach the combustion chamber through the injectors or injection valves. As a consequence, in an engine equipped with the injection system of the invention, there are also no emissions of uncombusted hydrocarbon compounds that would originate in fuel that reached the combustion chamber while the engine was off.
Since the pressure prevailing on the pressure side and the intake side of the high-pressure fuel pump is still high enough, while the engine is off, to reliably prevent the creation of vapor bubbles, when the engine is started the pressure required for injection on the pressure side of the high-pressure fuel pump is reached more quickly, and thus a faster start of the engine is made possible. Because of the faster start of the engine, overall emissions from the engine are further improved, and the electrical system is relieved as well. Furthermore, the relief device of the invention is simple in construction and does not require triggering by an electronic control unit of the injection system.
In variants of the invention, it is provided that the relief device includes a bypass line, connecting the intake side and pressure side of the high-pressure fuel pump, and/or a throttle, so that in a simple way, the advantages of the injection system of the invention can be achieved.
In further features of the invention, the throttle can be embodied as a notch in a valve member or in a valve seat of a check valve of the high-pressure fuel pump, or as a perforated baffle. The perforated baffle can be integrated for instance with a common rail of the injection system, or with a check valve on the pressure side of the high-pressure fuel pump.
In an especially advantageous feature of the invention, the check valve is embodied as a flat seat valve, with a counterpart plate that has a valve seat and with a valve plate that cooperates with the valve seat, so that upon closure of the seat valve, a pinch flow is created, which makes for constant cleaning of the notch acting as a throttle. This assures that for the entire service life of the injection system, the notch will not become plugged up and thus will remain functional. Moreover, this embodiment can be produced with high dimensional accuracy, and the cross section of the throttle is virtually constant over the entire service life. It has proved advantageous if the notch has a rectangular, oval or half-round cross section, which is produced especially by electrochemical machining or by a reshaping operation, such as stamping. By these means, high dimensional accuracy is achieved at low production cost.
Alternatively, the throttle can be integrated into a pressure regulating valve of the high-pressure fuel system. This is especially advantageous whenever the relief device of the invention is to be integrated into the injection system with only the slightest possible changes.
In a variant of the invention, it is provided that a pressure-holding device, which in particular can be embodied as a pressure regulating valve or as a check valve, is provided on a pressure side of the low-pressure pump. Using this pressure-holding device reliably and at little cost assures that even with the engine off, a pressure on the intake side and pressure side of the high-pressure fuel pump still remains that is greater than or equal to the delivery head of the low-pressure pump. The development of vapor bubbles can thus be reliably prevented even when the engine is off.